AE_ERR -> ANET_ERR in acceptUnixHandler().

No actual changes since the value is the same.
Accept multiple clients per iteration.

When the listening sockets readable event is fired, we have the chance
to accept multiple clients instead of accepting a single one. This makes
Redis more responsive when there is a mass-connect event (for example
after the server startup), and in workloads where a connect-disconnect
pattern is used often, so that multiple clients are waiting to be
accepted continuously.

As a side effect, this commit makes the LOADING, BUSY, and similar
errors much faster to deliver to the client, making Redis more
responsive when there is to return errors to inform the clients that the
server is blocked in an not interruptible operation.
Process events with processEventsWhileBlocked() when blocked.

When we are blocked and a few events a processed from time to time, it
is smarter to call the event handler a few times in order to handle the
accept, read, write, close cycle of a client in a single pass, otherwise
there is too much latency added for clients to receive a reply while the
server is busy in some way (for example during the DB loading).
Test: fixed scripting.tcl test false positive.
sdscatfmt() added to SDS library.

sdscatprintf() relies on printf() family libc functions and is sometimes
too slow in critical code paths. sdscatfmt() is an alternative which is:

1) Far less capable.
2) Format specifier uncompatible.
3) Faster.

It is suitable to be used in those speed critical code paths such as
CLIENT LIST output generation.
Added new sdscatfmt() %u and %U format specifiers.

This commit also fixes a bug in the implementation of sdscatfmt()
resulting from stale references to the SDS string header after
sdsMakeRoomFor() calls.
Use sdscatfmt() in getClientInfoString() to make it faster.
CLIENT LIST speedup via peerid caching + smart allocation.

This commit adds peer ID caching in the client structure plus an API
change and the use of sdsMakeRoomFor() in order to improve the
reallocation pattern to generate the CLIENT LIST output.

Both the changes account for a very significant speedup.
Cluster test: basic cluster nodes info access functions.
Cluster test: check that every node has a different ID.
Cluster test: auto-discovery to form full mesh.
Cluster test: config epoch conflict resolution.
Cluster / Sentinel test: instances count moved to run.tcl.
Cluster test: use 20 instances.

This makes tests a bit slower, but it is better to test things at a
decent scale instead of using just a few nodes, and for a few tests we
actually need so many nodes.
Cluster test: slots allocation.
CLUSTER SET-CONFIG-EPOCH implemented.

Initially Redis Cluster accepted that after cluster creation all the
nodes were at configEpoch 0, evolving from zero as failovers happen.

However later the semantic was made more strict in order to make sure a
cluster has always all the master nodes with a different configEpoch,
which is more robust in some corner case (especially resulting from
errors by the system administrator).

To assign different configEpochs to different nodes at startup was a
task performed naturally by the config conflicts resolution algorithm
(see the Cluster specification). However this works well only for small
clusters or when there are actually just a few collisions, since it is
designed for exceptional cases.

When a large cluster is created hundred of nodes can be at epoch 0, so
the conflict resolution code is slow to provide an unique config to each
node. For this reason this new command was introduced. It can be called
only when a node is totally fresh: no other nodes known, and configEpoch
set to zero, so it is safe even against misuses.

redis-trib will use the new command in order to start the cluster
already setting an incremental unique config to every node.
Cluster test: check for state=ok after slot allocation.
Cluster test: Tcl cluster library initial skeleton.
Cluster: Tcl cluster client key -> hashslot.
Cluster: Tcl cluster client: get nodes description.